9/11

or 9-11

[nahyn--i-lev-uh n] /ˈnaɪn ɪˈlɛv ən/

1.

September 11, 2001: the day on which Islamic terrorists, believed to be part of the Al-Qaeda network, hijacked four commercial airplanes and crashed two of them into the World Trade Center in New York City and a third one into the Pentagon in Virginia: the fourth plane crashed into a field in rural Pennsylvania.